Help Please =D

Discussion in 'General Electronics Chat' started by Howitzer491, Feb 27, 2009.

  1. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    Hi guys nd girls
    Im wanting to connect some leds up to my speakers for the awsome effect.
    ive tried today nd managed to get them to work but after a while a orange flash nd they were dead :confused:

    I connected up 3v blue leds to my 95w stereo, and ive done some research and im guesing i need to put in a resistor, im not to good with figuring out what components i will need. sooo
    if any one can help me with this i will be very gratefull
    Cheers:)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    LED's are sensitive to current. Once you put enough voltage across them, they have no resistance and burn up easily - as you have just found out. Besides affecting the quality of sound, they are hard to attach to a speaker, as you have no idea of how much voltage will be across them, so one resistor won't be adequate to control current.

    What people used to do was make a color organ. The device taps off the audio signal before the amp and uses filters to get response for different frequency bands. This link - http://www.discovercircuits.com/C/color-org.htm - has several circuits on it.

    Google turns up 17,500,000 hits, so there's lots out there, including kits and prebuilts.
     
  3. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    Cheers mate
    well, i used a volt meter to check the sterio and it was barely anything going through so i gues i will have to make a colour organ like u sujested
    Thanx:)
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    95W into 8 ohms is a voltage of positive 39V and negative 39V. An LED needs only a few volts and a current-limiting resistor in series will reduce the current and if it is correctly calculated then the LED does not burn out. But the absolute max reverse voltage for an LED is only 5V and the 39V from the amplifier will destroy it unless two LEDs are connected back-to-back.

    The problem with connecting LEDs to a speaker output is the volume control. If the volume is turned down then the LEDs will be very dim.
     
  5. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    I dont mind because if i have it quiet it wont flash all the time, but when i turn up the sound WOW XD haha
    hope i can get it to work.

    someone told me a 330 ohm resistor would stop the LED burning out?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Most LEDs have a max allowed continuous current of 30mA. If you have a 3.3V blue LED then for 25mA the resistor must be (39V - 3.3V)/25mA= 1428 ohms. Use 1.5k/1W.
    Connect two LEDs back-to-back and fed from the 1.5k resistor.
     
  8. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    Hi there again i did some more tests with the voltmeter and realised i was using dcv not acv duh (different voltmeter than im used to) lol and i tested on a bassy song and it maxed out at 20.2v i hope this info helps
     
  9. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    So what should you sujest for a 3.2v led running off 21v?
     
    Last edited: Feb 27, 2009
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    (21v-3.2v)/25mA = 17.8/0.025 = 712 Ohms.
    Closest standard E24 value is 750 Ohms. Chart: http://www.logwell.com/tech/components/resistor_values.html
    Resistor power dissipation = voltage x current = 17.8 x .025 = 0.445 Watts; doubled for reliability = 0.89 Watts.
    Use a minimum of a 1W resistor.
     
  11. Howitzer491

    Thread Starter New Member

    Feb 27, 2009
    6
    0
    Hi, me again.
    I replaced the 330 ohm resistor with a 1k resistor and its brighter but gets up to 4v max :O
    Should i use a 1.5k resistor?
     
    Last edited: Mar 2, 2009
Loading...